Technical Field
[0001] Embodiments of the present invention relate to an overvoltage protection device of
a variable-speed power system.
Background Art
[0002] A variable-speed pumped storage power system using a doubly-fed alternator is equipped
with an overvoltage suppression device in order to suppress an overvoltage in a secondary
circuit that is generated when a grid system failure has occurred and to protect main
circuit equipment, such as a generator-motor or a converter, from the overvoltage.
As an example, the overvoltage suppression device uses a large-current and short-time
rating short-circuit device that is configured by a large-capacity semiconductor element
such as a thyristor. In the variable-speed pumped storage power system, a function
is implemented whereby, when a short-circuit fault is removed by a grid system circuit
breaker after the occurrence of the grid system failure, a switching element of an
inverter of a secondary exciter is operated in such a way that a DC voltage of the
inverter is applied as a reverse voltage to the thyristor configuring the short-circuit
device of the overvoltage suppression device such that a current flowing through the
thyristor is brought to zero and the thyristor is forcibly turned off, the overvoltage
suppression device is freed early and reliably from a short-circuit state, the operation
of the secondary exciter is resumed, and the variable-speed pumped storage power system
is restored at high speed without being stopped (see, for example, Patent Literature
1). Hereinafter, control using this function is referred to as "reset control" of
the overvoltage suppression device.
[CITATION LIST]
[PATENT LITERATURE]
[0003] Patent Literature 1: Japanese Patent No.
3286049 (paragraphs [0026] to [0027], FIG. 1, etc.)
Disclosure of Invention
[0004] In the overvoltage suppression device described above, when reset control is performed
on the overvoltage suppression device in order to continue the operation of the variable-speed
pumped storage power system after the removal of a grid system failure, a semiconductor
element such as a thyristor that configures a short circuit receives a line-to-line
voltage after the completion of reset control at the time of the grid system failure.
Therefore, it is requested that the temperature of a joint of a semiconductor immediately
after resetting be lower than or equal to a temperature at which the joint can withstand
a secondary circuit voltage.
[0005] Therefore, in a case where reset control is performed, an allowable increase in temperature
is reduced in comparison with a case where reset control is not performed, and a maximum
current that can be applied to the semiconductor element of the overvoltage suppression
device is reduced. Accordingly, it is requested that the number of parallel circuits
of the semiconductor element be increased as needed.
[0006] Originally, reset control does not need to be performed on the overvoltage suppression
device so as to continue an operation at the time of a failure inside a power plant,
but needs to be performed at the time of a grid system failure on a high-voltage side
of a main transformer. Accordingly, a duty for reset control required for the semiconductor
element that configures the short-circuit device of the overvoltage suppression device
is to cope with a maximum failure current at the time of the grid system failure on
the high-voltage side of the main transformer. Reset control does not need to be performed
with respect to a larger fault current at the time of a failure inside a power plant
that occurs on a low-voltage side of the main transformer. Accordingly, even when
the temperature of the joint of the semiconductor increases, it is only necessary
to consider a resistance that enables a current to be applied in a short-circuit state.
[0007] However, in a case where the overvoltage suppression device is designed based on
the maximum failure current at the time of the grid system failure on the high-voltage
side of the main transformer, when a failure on the low-voltage side of the main transformer
fails to be determined and reset control is erroneously performed on the overvoltage
suppression device, equipment such as the short-circuit device including the semiconductor
element may be damaged. Therefore, conventionally, the overvoltage suppression device
is designed to increase in size, for example, by increasing the number of parallel
circuits of the semiconductor element. By doing this, the equipment is not damaged
even when reset control is performed with respect to a maximum failure current at
the time of the occurrence of a three-phase short-circuit failure on the low-voltage
side of the main transformer. The overcurrent resistance of a semiconductor having
a small thermal capacity is small. Therefore, the size of the equipment increases
even in a short-time operating duty, and this results in an increase in a price of
the short-circuit device, the size of the device, and the space of a power plant.
[0008] The present invention is made in order to solve the problems described above, and
it is an object of the present invention to provide an overvoltage protection device
of a variable-speed power system that performs appropriate reset control such that
equipment is not damaged, and that achieves a reduction in the number of parallel
circuits of a semiconductor element that configures a short-circuit device, a reduction
in a manufacturing cost, a reduction in the size of a device, a reduction in the size
of a building, or the like.
[0009] According to an embodiment, there is provided an overvoltage protection device of
a variable-speed power system that includes a transformer for which a high-voltage
side is connected to a grid system, a winding induction device in which a stator winding
is connected to a circuit on a low-voltage side of the transformer, an AC exciter
that is connected to a circuit of a rotor winding of the winding induction device,
an overvoltage suppression device that connects each phase via a short-circuit device
connected to a circuit between the AC exciter and the rotor winding or via the short-circuit
device and a resistor, and a control device that performs control to forcibly release
an operation of the overvoltage suppression device, the overvoltage protection device
of the variable-speed power system comprising: a detection unit configured to detect
a prescribed phenomenon that occurs when a failure has occurred on the high-voltage
side of a main transformer or when a failure has not occurred on the low-voltage side
of the main transformer or inside the main transformer or to detect that the prescribed
phenomenon has not occurred; and a control unit configured to determine based on a
detection result of the detection unit whether control to forcibly release the operation
of the overvoltage suppression device is to be permitted to be performed.
Brief Description of Drawings
[0010]
FIG. 1 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to a first embodiment.
FIG. 2 is a configuration diagram mainly showing a functional configuration related
to an overvoltage protection device of the variable-speed pumped storage power system
according to the first embodiment.
FIG. 3 is a diagram showing an example of an internal configuration of a reset control
locking condition determination unit in FIG. 2.
FIG. 4 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to a fifth embodiment.
FIG. 5 is a configuration diagram mainly showing a functional configuration related
to an overvoltage protection device of the variable-speed pumped storage power system
according to the fifth embodiment.
FIG. 6 is a diagram showing an example of an internal configuration of a reset control
locking condition determination unit in FIG. 5.
FIG. 7 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to a sixth embodiment.
FIG. 8 is a configuration diagram mainly showing a functional configuration related
to an overvoltage protection device of the variable-speed pumped storage power system
according to the sixth embodiment.
FIG. 9 is a diagram showing an example of an internal configuration of a reset control
locking condition determination unit in FIG. 8.
FIG. 10 is a diagram explaining processing of a threshold processing circuit in FIG.
9.
[0011] Mode for Carrying Out the Invention Embodiments are described below with reference
to the accompanying drawings.
(First Embodiment)
[0012] Initially, a first embodiment is described.
[0013] FIG. 1 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to the first embodiment.
[0014] As shown in FIG. 1, a power plant and power-plant switchgear P includes a power-plant
high-voltage switchgear 100 connected to a grid system S, and a variable-speed pumped
storage power system 200 connected to the power-plant high-voltage switchgear 100
via a power cable C and a main transformer M. A low-voltage side of the main transformer
M is equivalent to an internal circuit of the power plant. The main transformer M
may be configured as a portion of the variable-speed pumped storage power system.
[0015] The power-plant high-voltage switchgear 100 includes circuit breakers 101 and 102
that respectively open and close power lines connected, for example, to a No. 1 power
transmission line and a No. 2 power transmission line on a side of the grid system
S. The power-plant high-voltage switchgear 100 also includes a circuit breaker 103
that opens and closes a power line connected to the power cable C on a side of the
main transformer M.
[0016] The variable-speed pumped storage power system includes: a pump turbine 1; a variable-speed
generator-motor (a winding induction machine) 2 implemented by a doubly-fed alternator;
a self-excitation secondary exciter (an AC exciter) 3 that is connected to a secondary
winding of the variable-speed generator-motor 2 and applies an AC of a variable frequency
and that is implemented by a frequency converter, the self-excitation secondary exciter
3 including a converter 3A and an inverter 3B; a control device 4 that controls a
voltage, a current, a frequency, and a phase of the AC output by the secondary exciter
3 and that also controls, for example, the opening/closing of switchgears 5A and 5B
and the like; a parallel circuit breaker 5A and a phase inversion disconnector 5B
for system interconnection that are connected to a stator winding side of the variable-speed
generator-motor 2; a start/brake disconnector 6 that causes three-phase short-circuiting
at a stator winding terminal of the variable-speed generator-motor 2; an excitation
transformer 7 that generates an AC power supply to be used by the secondary exciter
3; an excitation circuit breaker 8 that interrupts AC power supplied to the excitation
transformer 7; a primary circuit voltage detector (an instrument transformer) 9 that
measures a voltage of a primary circuit on the stator winding side of the variable-speed
generator-motor 2; a secondary circuit current detector 10 that measures a current
flowing through a secondary circuit on a rotor winding side of the variable-speed
generator-motor 2; a secondary circuit voltage detector 11 that measures a voltage
of the secondary circuit on the rotor winding side of the variable-speed generator-motor
2 or a DC link voltage of the secondary exciter 3; a short-circuit current detector
12 that measures a three-phase short-circuit current in the secondary circuit on the
rotor winding side of the variable-speed generator-motor 2; an overvoltage suppression
device 13 using a large-current and short-time rating short-circuit device configured
by a large-capacity semiconductor element such as a thyristor; and others. The overvoltage
suppression device 13 connects respective phases via a short-circuit device connected
to a circuit between the secondary exciter 3 and a rotor winding of the variable-speed
generator-motor 2 or via the short-circuit device and a resistor.
[0017] FIG. 2 is a configuration diagram mainly showing a functional configuration related
to an overvoltage protection device of the variable-speed pumped storage power system
according to the present embodiment. The same reference numbers are given to the same
components as those in FIG. 1.
[0018] An overvoltage protection device 20 of this variable-speed pumped storage power system
includes at least a portion of the overvoltage suppression device 13 and the control
device 4 described above, and also includes a measurement device, a detector, and
the like that measure and detect an electric quantity such as a current or a voltage
of each part, element temperature, and the operation states of various protection
relays. Further, the overvoltage protection device 20 includes a detection unit (this
is equivalent, for example, to a "reset control locking condition determination unit
27" described later) and a control unit (this is equivalent, for example, to "logic
circuits L1 and L2, switching control circuits F1 and F2, and switches SW1 and SW2"
described later). The detection unit detects a prescribed phenomenon that occurs when
a failure has occurred on a high-voltage side of the main transformer M or when a
failure has not occurred on the low-voltage side of the main transformer M or inside
the main transformer M, or the detection unit detects that the prescribed phenomenon
has not occurred. The control unit determines based on a detection result of the detection
unit whether control to forcibly release an operation of the overvoltage suppression
device will be permitted to be performed.
[0019] The overvoltage suppression device 13 includes a short-circuit device 16, and the
short-circuit device 16 includes thyristors 14 that each are connected between phases
of a secondary winding of the variable-speed generator-motor 2, and thyristor current
detectors 15 that detect currents of the thyristors 14. The thyristor current detectors
15 are equivalent, for example, to the short-circuit current detector 12 described
above. In addition, the overvoltage suppression device 13 is equipped with an overvoltage
suppression device gate circuit 17 that gives a gate signal that controls the firing/extinguishing
of each of the thyristors 14 to a gate of each of the thyristors 14.
[0020] The control device 4 performs appropriate reset control so that equipment such as
the short-circuit device 16 including a semiconductor element is not damaged. As an
example, when a value of an applied current of the short-circuit device 16 during
a short-circuit operation is smaller than or equal to a current specified value that
is specified in advance, an elapsed time of the short-circuit operation of the short-circuit
device 16 is greater than or equal to an operation continuation time specified value
that is specified in advance (or the disconnection of a circuit breaker connected
to the grid system S is completed), and a measured value of an electric quantity obtained
from the rotor winding side or the stator winding side of the variable-speed generator-motor
2 satisfies a given condition, the control device 4 performs control to forcibly release
a short-circuit state of the short-circuit device 16. Specifically, the given condition
means, for example, that a value of a "voltage on the low-voltage side of the main
transformer M" (a voltage on the stator winding side) during a prescribed determination
period before control to forcibly release the short-circuit state of the short-circuit
device 16 is performed is not smaller than or equal to a voltage specified value that
is specified in advance (or does not fall below the voltage specified value).
[0021] The control device 4 described above is configured by an inverter gate signal generator
21, a normal operation gate controller 22, a reset control gate controller 23, a secondary
circuit overvoltage determination unit 24, a short-circuit state detector 25, a reset
permission determination unit 26, a reset control locking condition determination
unit 27, switches SW1 and SW2, logic circuits L1 and L2, switching control circuits
F1 and F2, and the like.
[0022] The inverter gate signal generator 21 generates a gate signal that drives the inverter
3B of the secondary exciter 3.
[0023] The normal operation gate controller 22 outputs a signal that controls a gate signal
for normal operation.
[0024] The reset control gate controller 23 outputs a signal that controls a gate signal
for reset control.
[0025] The secondary circuit overvoltage determination unit 24 determines whether an overvoltage
of a prescribed level or more has occurred in the secondary circuit on the rotor winding
side of the variable-speed generator-motor 2. When the secondary circuit overvoltage
determination unit 24 determines that the overvoltage has occurred, the secondary
circuit overvoltage determination unit 24 outputs a signal indicating the occurrence
of the overvoltage, namely, a signal instructing an overvoltage protection (OVP) operation
(for example, a signal of the value "1").
[0026] The short-circuit state detector 25 detects a state of the short-circuit device 16
based on the value of the applied current during the short-circuit operation of the
short-circuit device 16, namely, current values of the thyristors 14. When the short-circuit
state detector 25 detects that the short-circuit device 16 is short-circuited, the
short-circuit state detector 25 outputs a signal indicating this fact (for example,
a signal of the value "1").
[0027] The reset permission determination unit 26 determines based on the current values
of the thyristors 14 whether the resetting of the OVP operation will be permitted.
When the current values of the thyristors 14 are smaller than or equal to a current
specified value that is specified in advance, the reset permission determination unit
26 outputs a signal indicating the permission of resetting (for example, a signal
of the value "1").
[0028] The reset control locking condition determination unit 27 determines whether a condition
for locking reset control (a condition for not performing reset control) is satisfied
based on a voltage V
L of each phase of a three-phase AC detected by a primary circuit voltage detector
9 and the signal output from the secondary circuit overvoltage determination unit
24. When the reset control locking condition determination unit 27 determines that
the condition is satisfied, the reset control locking condition determination unit
27 outputs a signal instructing the locking of reset control (for example, a signal
of the value "1"). When the reset control locking condition determination unit 27
determines that the condition is not satisfied, the reset control locking condition
determination unit 27 outputs a signal indicating that reset control will not be locked
(for example, a signal of the value "0").
[0029] The switch SW1 switches the supply/stop of a command to generate the gate signal
for normal operation in accordance with the switching of the ON/OFF state.
[0030] The switch SW2 switches the supply/stop of a command to generate the gate signal
for reset control in accordance with the switching of the ON/OFF state.
[0031] The logic circuit L1 receives an inverted signal of the signal output from the secondary
circuit overvoltage determination unit 24, the signal output from the short-circuit
state detector 25, the signal output from the reset permission determination unit
26, and an inverted signal of the signal output from the reset control locking condition
determination unit 27. When an AND condition of each of the received signals is satisfied
(for example, when the values of all of the signals are "1"), the logic circuit L1
outputs a signal instructing that reset control will be performed (for example, a
signal of the value "1").
[0032] The logic circuit L2 receives the signal output from the logic circuit L1 and an
inverted signal of the signal output from the short-circuit state detector 25. When
an AND condition of each of the received signals is satisfied (for example, when the
values of both signals are "1"), the logic circuit L2 outputs a signal instructing
that reset control will be released (and normal operation will be performed) (for
example, a signal of the value "1").
[0033] The switching control circuit F1 turns on the switch SW1 in accordance with the signal
output from the logic circuit L2 (for example, a signal of the value "1"), and turns
off the switch SW1 in accordance with the signal output from the secondary circuit
overvoltage determination unit 24 (for example, a signal of the value "1").
[0034] The switching control circuit F2 turns on the switch SW2 in accordance with the signal
output from the logic circuit L1, and turns off the switch SW2 in accordance with
the signal output from the logic circuit L2.
[0035] FIG. 3 is a diagram showing an example of an internal configuration of the reset
control locking condition determination unit 27.
[0036] The reset control locking condition determination unit 27 includes a one-shot timer
31, an arithmetic circuit 32, a threshold processing circuit 33, a logic circuit 34,
a NOT circuit 35, a switching control circuit 36, and the like.
[0037] The one-shot timer 31 measures a specified time (a determination time specified value)
from a point in time at which the signal instructing the overvoltage protection (OVP)
operation (for example, a signal of the value "1") is input from the secondary circuit
overvoltage determination unit 24 to a point in time at which reset control is performed.
The one-shot timer 31 outputs a prescribed signal (for example, a signal of the value
"1") until the specified time elapses from the point in time at which the signal is
input.
[0038] The arithmetic circuit 32 calculates an absolute value of a voltage vector from the
voltage V
L of each phase of the three-phase AC that has been output from the primary circuit
voltage detector 9, and outputs the absolute value.
[0039] When the value output from the arithmetic circuit 32 is a preset threshold (for example,
40% of a rated voltage of the primary circuit) (or when the value is smaller than
the preset threshold), the threshold processing circuit 33 outputs a prescribed signal
(for example, a signal of the value "1"). When the value output from the arithmetic
circuit 32 is not the preset threshold (or is not smaller than the preset threshold),
the threshold processing circuit 33 outputs a prescribed signal that is different
from the signal above (for example, a signal of the value "0").
[0040] The threshold is set from the viewpoint of not hindering the performance of reset
control when a failure has occurred on the high-voltage side of the main transformer
M and of obviating the flowing of a current that may damage equipment such as the
short-circuit device 16 including the semiconductor element in a case where reset
control is performed when a failure has occurred on the low-voltage side of the main
transformer M or inside the main transformer M. Accordingly, the threshold is set,
for example, in such a way that a failure occurrence point at the time when a failure
has occurred (whether a failure has occurred on the high-voltage side of the main
transformer M or has occurred on the low-voltage side of the main transformer M or
inside the main transformer M) can be identified from an output value of the threshold
processing circuit 33.
[0041] More specifically, when the failure occurrence point is located on the high-voltage
side of the main transformer M, the threshold is set in such a way that the value
output from the arithmetic circuit 32 (namely, the absolute value of the voltage vector
calculated from the voltage V
L of each phase) exceeds the threshold or is greater than or equal to the threshold.
When the failure occurrence point is located on the low-voltage side of the main transformer
M or inside the main transformer M, the threshold is set in such a way that the value
output from the arithmetic circuit 32 (namely, the absolute value of the voltage vector
calculated from the voltage V
L of each phase) is smaller than or equal to the threshold or is smaller than the threshold.
[0042] By setting the threshold as described above, the reset control locking condition
determination unit 27 can detect based on the output value of the threshold processing
circuit 33 that a failure has occurred on the high-voltage side of the main transformer
M (or that a failure has not occurred on the low-voltage side of the main transformer
M or inside the main transformer M), or that a failure has occurred on the low-voltage
side of the main transformer M or inside the main transformer M (or that a failure
has not occurred on the high-voltage side of the main transformer M) .
[0043] A specific example of setting the threshold is given below.
[0044] As an example, when a failure has occurred on the high-voltage side of the main transformer
M, a voltage V
L that is detected by the primary circuit voltage detector 9 on the low-voltage side
of the main transformer M at the moment of occurrence is obtained according to the
following equation.
where
Ve: internal induced voltage (pu value) of generator;
Xg: internal impedance (pu value) of generator;
Xt: impedance (pu value) of main transformer; and
Xf: impedance (pu value) to fault point.
[0045] By using equation (1) described above, a position of a failure (a fault point) can
be determined from an amount of a decrease in V
L.
[0046] Here, assume that Xg is 20% and Xt is 15%.
[0047] When a failure has occurred in a position closest to the high-voltage side of the
main transformer M, it can be considered that Xf = 0. Therefore, the voltage V
L is about "40% of the rated voltage of the primary circuit" according to equation
(1). Stated another way, it is understood that this value is a value in a case where
the voltage decreases most at the moment of the occurrence of a failure on the high-voltage
side of the main transformer M. Therefore, when the voltage is lower than this value,
a failure has not occurred on the high-voltage side of the main transformer M, but
a failure has occurred on the low-voltage side of the main transformer M or inside
the main transformer M. Accordingly, by applying this value to the threshold of the
threshold processing circuit 33, a failure occurrence point at the time when a failure
has occurred (whether a failure has occurred on the high-voltage side of the main
transformer M, or has occurred on the low-voltage side of the main transformer M or
inside the main transformer M) can be identified.
[0048] A basic idea has been described above. In an actual generator-motor, excitation control
is performed at high speed, and the internal induced voltage Ve changes during the
occurrence of a failure. Therefore, identification can be performed accurately by
determining the threshold described above based on a result obtained by calculating
a value of a voltage generated in an actual machine by using a computer analysis program
such as the electro-magnetic transients program (EMTP).
[0049] When an AND condition between the signal output from the one-shot timer 31 and the
signal output from the threshold processing circuit 33 is satisfied (for example,
when the values of both signals are "1"), the logic circuit 34 outputs a signal instructing
that reset control will be locked.
[0050] The NOT circuit 35 receives a signal indicating an opening/closing state of the parallel
circuit breaker 5A. As an example, in a process in which the variable-speed pumped
storage power system 200 is stopped for protection by a not-illustrated protection
relay system after reset control is locked, when the NOT circuit 35 receives a signal
indicating an opening state of the parallel circuit breaker 5A (for example, a signal
of the value "0"), the NOT circuit 35 inverts the signal, and outputs a signal after
inversion (for example, a signal of the value "1") as a signal instructing that locking
will be released.
[0051] When the switching control circuit 36 receives the signal instructing that reset
control will be locked (for example, a signal of the value "1") from the logic circuit
34, the switching control circuit 36 outputs the signal instructing that reset control
will be locked (for example, a signal of the value "1") with no change. When the switching
control circuit 36 receives the signal instructing that locking will be released (for
example, a signal of the value "1") from the NOT circuit 35, the switching control
circuit 36 outputs a signal that does not instruct that reset control will be locked
(for example, a signal of the value "0").
[0052] As an example, a case is considered where a grid system failure has occurred in the
No. 1 power transmission line or the No. 2 power transmission line of the grid system
S during normal operation in the system configuration described above. When a failure
current including a transient DC component is generated in the secondary circuit on
the rotor winding side of the variable-speed generator-motor 2 due to this grid system
failure, a DC capacitor is charged through an anti-parallel diode of the inverter
3B, and an overvoltage is generated in the secondary circuit.
[0053] A value of a voltage detected by the secondary circuit voltage detector 11 is transmitted
to the secondary circuit overvoltage determination unit 24. When the secondary circuit
overvoltage determination unit 24 determines that an overvoltage was generated in
the secondary circuit, the secondary circuit overvoltage determination unit 24 stops
the generation of the gate signal for normal operation under the control of the normal
operation gate controller 22 via the switching control circuit F1 and the switch SW1.
In addition, the secondary circuit overvoltage determination unit 24 short-circuits
the short-circuit device 16 via the overvoltage suppression device gate circuit 17,
and flows the failure current into the short-circuit device 16 so as to suppress the
overvoltage of the secondary circuit.
[0054] When the failure current is attenuated with time and the reset permission determination
unit 26 detects that the failure current becomes lower than or equal to a current
that can be controlled by the inverter 3B, the reset permission determination unit
26 outputs the signal indicating the permission of resetting (for example, a signal
of the value "1"). At this time, the short-circuit state detector 25 detects the state
of the short-circuit device 16, and outputs a signal indicating that the short-circuit
device 16 is short-circuited (for example, a signal of the value "1"). The reset control
locking condition determination unit 27 outputs a signal indicating that a condition
for locking reset control is not satisfied (for example, a signal of the value "0"),
because the absolute value of the voltage vector obtained from the voltage V
L of each phase exceeds a threshold that is specified in advance (or is greater than
or equal to the threshold). In addition, the secondary circuit overvoltage determination
unit 24 outputs a signal indicating that an overvoltage has not been generated (for
example, a signal of the value "0").
[0055] In this case, an AND condition of each of the signals input to the logic circuit
L1 is satisfied (for example, values of all of the signals are "1"), and therefore,
the logic circuit L1 outputs a signal instructing that reset control will be performed
(for example, a signal of the value "1"), and causes the inverter 3B to be supplied
with the gate signal for reset control under the control of the reset control gate
controller 23, via the switching control circuit F2 and the switch SW2. By doing this,
the reset control gate controller 23 controls the inverter gate signal generator 21
to provide the gate signal such that a reverse voltage is applied from the inverter
3B to each of the thyristors 14 that are electrically connected, and the short-circuit
state of the short-circuit device 16 is released at high speed.
[0056] When the short-circuit state detector 25 detects that the short-circuit state of
the short-circuit device 16 has been released, the logic circuit L1 stops an instruction
to perform reset control. Meanwhile, the logic circuit L2 causes the generation of
the gate signal for reset control under the control of the reset control gate controller
23 to be stopped via the switching control circuit F2 and the switch SW2. The logic
circuit L2 also causes the generation of the gate signal for normal operation under
the control of the normal operation gate controller 22 to be resumed via the switching
control circuit F1 and the switch SW1. By doing this, the normal operation of the
secondary exciter 3 is resumed, the variable-speed pumped storage power system 200
is restored to normal operation after the removal of a failure, and operation is continued.
[0057] Next, a case is considered where a three-phase short-circuit failure has occurred
inside the variable-speed pumped storage power system 200 that is located on the low-voltage
side of the main transformer M in the system configuration described above. Description
of an operation common to an operation in the case of the grid system failure described
above is omitted, and a different operation is principally described.
[0058] When a three-phase short-circuit failure has occurred inside the variable-speed pumped
storage power system 200, the absolute value of the voltage vector obtained from the
voltage V
L of each phase is zero in the reset control locking condition determination unit 27
described above, and the absolute value is smaller than the threshold described above.
Therefore, the reset control locking condition determination unit 27 determines that
the condition for locking reset control was satisfied within a prescribed determination
period, and outputs an output signal indicating this fact (for example, a signal of
the value "1").
[0059] In this case, an AND condition of each of the signals input to the logic circuit
L1 is not satisfied (not all of the signals have the value "1"), and therefore, the
logic circuit L1 does not output a signal instructing that reset control will be performed
(for example, a signal of the value "1"), the states of the switching control circuit
F2 and the switch SW2 do not change, and reset control under the control of the reset
control gate controller 23 is locked.
[0060] In a process in which the variable-speed pumped storage power system 200 is stopped
for protection by the protection relay system after reset control is locked, when
the reset control locking condition determination unit 27 receives a signal indicating
an opening state of the parallel circuit breaker 5A (for example, a signal of the
value "0"), the reset control locking condition determination unit 27 outputs a signal
that does not instruct that reset control will be locked (for example, a signal of
the value "0"). By doing this, a locking state is released.
[0061] According to the first embodiment, as an example, even when a three-phase short-circuit
failure has occurred inside the variable-speed pumped storage power system 200 and
the temperature of semiconductor elements such as the thyristors 14 in the short-circuit
device 16 increases to exceed an allowable temperature during normal operation due
to a large short-circuit current, the semiconductor elements in the short-circuit
device 16 are not damaged due to the application of a voltage, and the semiconductor
elements can remain a short-circuit state without the release of the short-circuit
state. The variable-speed pumped storage power system 200 can be safely stopped by
the protection relay system that operates due to the short-circuit failure. In addition,
a current failure is not a grid system failure, and therefore the continuation of
the operation of the variable-speed pumped storage power system 200 after the removal
of the failure is not requested, and no hindrance occurs.
[0062] Further, the semiconductor elements such as the thyristors 14 are not damaged due
to the application of a voltage in a high-temperature state. Therefore, the design
of the overvoltage suppression device 13 can be determined based on a maximum failure
current at the time of a grid system failure. In a case where a function of reset
control is added, a reduction in the number of parallel circuits of the semiconductor
elements that configure the short-circuit device 16 of the overvoltage suppression
device 13, a reduction in a manufacturing cost, a reduction in the size of a device,
a reduction in the size of a building, and the like can be achieved.
[0063] In the present embodiment, in the reset control locking condition determination unit
27, the condition of a "prescribed determination period" before control is performed
to forcibly release the short-circuit state of the short-circuit device 16 is added
to conditions for the absolute value of the voltage vector obtained from the voltage
V
L of each phase that is detected by the primary circuit voltage detector 9.
[0064] Even when a transient DC component at the time of the occurrence of a grid system
failure is large and the short-circuit state of the short-circuit device 16 fails
to be forcibly released immediately after the removal of the failure, a maximum value
of a failure current at the time of the grid system failure is smaller than or equal
to a set maximum current value of the short-circuit device 16. Therefore, the temperature
of the semiconductor elements is smaller than or equal to an allowable value, and
reset control may be performed. As an example, when the circuit breaker 101 of the
power-plant high-voltage switchgear 100 is opened due to a grid system failure that
has occurred at a terminal connecting the No. 1 power transmission line of FIG. 1
such that the failure is removed, the transfer of power is resumed between the grid
system that has been restored to a voltage of the No. 2 power transmission line after
the removal of the failure and the power system. A voltage may decrease again according
to a current operation state, and the value of the voltage may become smaller than
a threshold set for the determination of the grid system failure. This results in
the unnecessary locking of reset control without any ingenuity. Accordingly, as an
example, when the one-shot timer 31 uses the "prescribed determination period" as
a time period until the grid system failure is removed, as described above, the locking
of reset control due to a decrease in a voltage at this time can be prevented. In
this case, when an applied current of the short-circuit device 16 is attenuated to
be lower than or equal to a current value specified in advance after a given time
period such that the reset condition is satisfied, the reset control locking condition
determination unit 27 does not output a reset locking signal. Therefore, reset control
can be performed, and the operation of the variable-speed pumped storage power system
200 can be continued.
[0065] In a case where the short-circuit state of the short-circuit device fails to be forcibly
released immediately after the removal of the grid system failure, when a power generation
facility may be stopped, a time period during which the reset control locking condition
determination unit 27 determines whether control will be performed to forcibly release
the short-circuit state of the short-circuit device 16 may be set to a time period
before the control is performed.
[0066] In this case, the reset control locking condition determination unit 27 is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage
side of the main transformer M or when a failure has not occurred on the low-voltage
side of the main transformer M or inside the main transformer M (in this case, a fact
that the value of a voltage on the low-voltage side of the main transformer M (a voltage
on the stator winding side) is not lower than or equal to a voltage specified value
that is specified in advance or is not lower than the voltage specified value during
a time period before control is performed to forcibly release the short-circuit state
of the short-circuit device 16 (for example, a time period until an output of 1 is
output from the reset permission determination unit 26), or the reset control locking
condition determination unit 27 is configured to detect that the prescribed phenomenon
has not occurred. In the logic circuits L1 and L2, the switching control circuits
F1 and F2, the switches SW1 and SW2, and the like, whether control to forcibly release
the operation of the overvoltage suppression device 13 will be permitted to be performed
is determined based on the detection result. Control to forcibly release the operation
of the overvoltage suppression device 13 is performed under a condition wherein the
phenomenon described above has been detected.
[0067] It is desirable that the short-circuit device 16 be configured to have a current
capacity that is smaller than a current capacity that enables control to forcibly
release the short-circuit state of the short-circuit device 16 to be performed on
a maximum secondary winding current at the time of a three-phase short-circuit failure
on the low-voltage side of the main transformer M, and configured to have a capacity
that is greater than or equal to a greater one of a current capacity that enables
a current to be applied until the generator-motor can be stopped without performing
control to forcibly release the short-circuit state of the short-circuit device 16
on the maximum secondary winding current at the time of the three-phase short-circuit
failure on the low-voltage side of the main transformer M or a current capacity that
enables control to forcibly release the short-circuit state of the short-circuit device
16 to be performed on a maximum secondary winding current at the time of a three-phase
short-circuit failure on the high-voltage side of the main transformer M. By employing
the configuration described above, the size of the entirety of the overvoltage suppression
device 13 including the short-circuit device 16 can be reduced without damaging equipment
even when reset control is performed on the maximum failure current at the time of
the occurrence of the three-phase short-circuit failure on the low-voltage side of
the main transformer M, and a reduction in a cost and a reduction in the space of
a power plant can be achieved.
(Second Embodiment)
[0068] A second embodiment is described next.
[0069] In the description below, description of a portion common to the first embodiment
is omitted, and a portion different from the first embodiment is principally described.
[0070] In the first embodiment described above, a fact that the value of the "voltage on
the low-voltage side of the main transformer M" during a period before control is
performed to forcibly release the short-circuit state of the short-circuit device
16 is not smaller than or equal to a voltage specified value that is specified in
advance (or does not fall below the voltage specified value) is one condition for
performing reset control. Whereas, in the second embodiment, a fact that the value
of a "rotor winding current" (an arm current (an OVP arm current) at a winding terminal
of a rotor winding of a variable-speed generator-motor 2 or on a side of a secondary
exciter 3) during a period before control is performed to forcibly release the short-circuit
state of the short-circuit device 16 is not greater than or equal to a current specified
value that is specified in advance (or does not exceed the current specified value
is one condition for performing reset control.
[0071] This is implemented by configuring a reset control locking condition determination
unit 27 to be provided with a value measured by a detector 12 or 15 that detects the
arm current (the OVP arm current) at the winding terminal of the rotor winding of
the variable-speed generator-motor 2 or on the side of the secondary exciter 3 instead
of the value of a voltage V
L of each phase, and by appropriately changing the design of the arithmetic expression
of an arithmetic circuit 32 and the threshold of a threshold processing circuit 33.
[0072] In this case, the reset control locking condition determination unit 27 is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on a high-voltage
side of the main transformer M or when a failure has not occurred on a low-voltage
side of the main transformer M or inside the main transformer M (in this case, a fact
that the value of a rotor winding current during a period before control is performed
to forcibly release the short-circuit state of the short-circuit device 16 is not
greater than or equal to a current specified value that is specified in advance or
does not exceed the current specified value), or the reset control locking condition
determination unit 27 is configured to detect that the prescribed phenomenon has not
occurred. In logic circuits L1 and L2, switching control circuits F1 and F2, switches
SW1 and SW2, and the like, whether control to forcibly release the operation of an
overvoltage suppression device 13 will be permitted to be performed is determined
based on the detection result. Control to forcibly release the operation of the overvoltage
suppression device 13 is performed under a condition wherein the phenomenon described
above has been detected.
[0073] According to the second embodiment, also in a configuration using the "rotor winding
current" in locking condition determination, effects similar to effects in the first
embodiment can be obtained.
(Third Embodiment)
[0074] A third embodiment is described next.
[0075] In the description below, description of a portion common to the first embodiment
is omitted, and a portion different from the first embodiment is principally described.
[0076] In the first embodiment described above, a fact that the value of the "voltage on
the low-voltage side of the main transformer M" during a period before control is
performed to forcibly release the short-circuit state of the short-circuit device
16 is not smaller than or equal to a voltage specified value that is specified in
advance (or does not fall below the voltage specified value) is one condition for
performing reset control. Whereas, in the third embodiment, a fact that the "temperature
of a prescribed element" (the temperature of semiconductor elements such as thyristors
14 that are included in a short-circuit device 16 or the temperature of semiconductor
elements included in a secondary exciter 3) during a period before control is performed
to forcibly release the short-circuit state of the short-circuit device 16 is not
higher than or equal to an element-temperature specified value that is specified in
advance or does not exceed the element-temperature specified value is one condition
for performing reset control.
[0077] This is implemented by configuring a reset control locking condition determination
unit 27 to be provided with a value measured by a detector (not illustrated) that
detects the temperature of the semiconductor elements such as the thyristors 14 that
are included in the short-circuit device 16 or the temperature of the semiconductor
elements included in the secondary exciter 3 instead of the value of a voltage V
L of each phase, and by appropriately changing the design of the arithmetic expression
of an arithmetic circuit 32 and the threshold of a threshold processing circuit 33.
[0078] In this case, the reset control locking condition determination unit 27 is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on a high-voltage
side of a main transformer M or when a failure has not occurred on a low-voltage side
of the main transformer M or inside the main transformer M (in this case, a fact that
the temperature of a prescribed element during a period before control is performed
to forcibly release the short-circuit state of the short-circuit device 16 is not
higher than or equal to an element-temperature specified value that is specified in
advance or does not exceed the element-temperature specified value), or the reset
control locking condition determination unit 27 is configured to detect that the prescribed
phenomenon has not occurred. In logic circuits L1 and L2, switching control circuits
F1 and F2, switches SW1 and SW2, and the like, whether control to forcibly release
the operation of an overvoltage suppression device 13 will be permitted to be performed
is determined based on the detection result. Control to forcibly release the operation
of the overvoltage suppression device 13 is performed under a condition wherein the
phenomenon described above has been detected.
[0079] According to the third embodiment, also in a configuration using the "temperature
of a prescribed element" in locking condition determination, effects similar to effects
in the first embodiment can be obtained.
(Fourth Embodiment)
[0080] A fourth embodiment is described next.
[0081] In the description below, description of a portion common to the first embodiment
is omitted, and a portion different from the first embodiment is principally described.
[0082] In the first embodiment described above, a fact that the value of the "voltage on
the low-voltage side of the main transformer M" during a period before control is
performed to forcibly release the short-circuit state of the short-circuit device
16 is not smaller than or equal to a voltage specified value that is specified in
advance (or does not fall below the voltage specified value) is one condition for
performing reset control. Whereas, in the fourth embodiment, a fact that a point of
the occurrence of an electric failure that results in the occurrence of an overvoltage
is located on a grid-system side which is away from a winding terminal on a high-voltage
side of a main transformer is one condition for performing reset control.
[0083] This is implemented by configuring a reset control locking condition determination
unit 27 to be provided with an operation signal of, for example, a segment protection
relay (such as a generator current differential relay, a principal circuit bus current
differential relay, or a main transformer differential current relay) that detects
a failure occurrence point of a failure in a plant or a failure of the grid system
that stops for protection a variable-speed pumped storage power system including a
power-plant high-voltage switchgear or a value indicating a failure occurrence point
that is specified by the operation signal, instead of the value of a voltage V
L of each phase, and by changing the design of the arithmetic expression of an arithmetic
circuit 32 and a threshold processing circuit 33 in such a way that the threshold
processing circuit 33 outputs a signal that locks reset control (for example, a signal
of the value "1") in the case of a failure that causes the variable-speed pumped storage
power system to be stopped for protection.
[0084] In this case, the reset control locking condition determination unit 27 is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on a high-voltage
side of a main transformer M or when a failure has not occurred on a low-voltage side
of the main transformer M or inside the main transformer M (in this case, a fact that
a prescribed protection relay device does not operate during a period before control
is performed to forcibly release the short-circuit state of a short-circuit device
16), or the reset control locking condition determination unit 27 is configured to
detect that the prescribed phenomenon has not occurred. In logic circuits L1 and L2,
switching control circuits F1 and F2, switches SW1 and SW2, and the like, whether
control to forcibly release the operation of an overvoltage suppression device 13
will be permitted to be performed is determined based on the detection result. Control
to forcibly release the operation of the overvoltage suppression device 13 is performed
under a condition wherein the phenomenon described above has been detected.
[0085] According to the fourth embodiment, also in a configuration using the operation of
a generator current differential relay, a principal circuit bus current differential
relay, a main transformer differential current relay, or the like in reset control
locking condition determination, effects similar to effects in the first embodiment
can be obtained.
(Fifth Embodiment)
[0086] A fifth embodiment is described next.
[0087] In the description below, description of a portion common to the first embodiment
is omitted, and a portion different from the first embodiment is principally described.
[0088] FIG. 4 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to the fifth embodiment. FIG.
5 is a configuration diagram mainly showing a functional configuration related to
an overvoltage protection device 20 of the variable-speed pumped storage power system
according to the fifth embodiment. FIG. 6 is a diagram showing an example of an internal
configuration of a reset control locking condition determination unit 27' in FIG.
5. The same reference numbers are given to the same components as those in FIG. 1
to FIG. 3.
[0089] In the first embodiment described above, a fact that the value of the "voltage on
the low-voltage side of the main transformer M" during a period before control is
performed to forcibly release the short-circuit state of the short-circuit device
16 is not smaller than or equal to a voltage specified value that is specified in
advance (or does not fall below the voltage specified value) is one condition for
performing reset control. Whereas, in the fifth embodiment, a fact that the value
of a "voltage on a high-voltage side of a main transformer M" during a period before
control is performed to forcibly release the short-circuit state of a short-circuit
device 16 is not smaller than or equal to a voltage specified value that is specified
in advance (or does not fall below the voltage specified value) is one condition for
performing reset control.
[0090] This is implemented by configuring the reset control locking condition determination
unit 27' to be provided with the value of a voltage V
S of each phase detected by a voltage detector 104 on the high-voltage side of the
main transformer M instead of the value of a voltage V
L of each phase detected by a primary circuit voltage detector 9 on a low-voltage side
of the main transformer M, and by appropriately changing the design of the arithmetic
expression of an arithmetic circuit 32' and the threshold of a threshold processing
circuit 33'.
[0091] In this case, the reset control locking condition determination unit 27' is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage
side of the main transformer M or when a failure has not occurred on the low-voltage
side of the main transformer M or inside the main transformer M (in this case, a fact
that the value of a voltage on the high-voltage side of the main transformer M during
a period before control is performed to forcibly release the short-circuit state of
the short-circuit device 16 is not smaller than or equal to a voltage specified value
that is specified in advance or does not fall below the voltage specified value),
or the reset control locking condition determination unit 27' is configured to detect
that the prescribed phenomenon has not occurred. In logic circuits L1 and L2, switching
control circuits F1 and F2, switches SW1 and SW2, and the like, whether control to
forcibly release the operation of an overvoltage suppression device 13 will be permitted
to be performed is determined based on the detection result. Control to forcibly release
the operation of the overvoltage suppression device 13 is performed under a condition
wherein the phenomenon described above has been detected.
[0092] In some power plants, as requirements for continuing operation at the time of a grid
system failure, it is not requested that a voltage on a high-voltage side of a main
transformer be 0V (namely, a complete short-circuit at a terminal connecting a grid
system), and the value of the voltage on the high-voltage side of the main transformer
M that is obtained from the value of a voltage V
S of each phase detected by a voltage detector 104 is specified to be greater than
or equal to a certain value (for example, "20% of a rated voltage of a primary circuit").
In this case, by applying this value to the threshold of a threshold processing circuit
33 and setting this value as one condition for performing reset control, an overvoltage
protection device can be provided that has a necessary and sufficient capacity in
a case where the continuation of operation is requested, similarly to the first embodiment.
[0093] According to the fifth embodiment, also in a configuration using the "voltage on
the high-voltage side of the main transformer M" in locking condition determination,
effects similar to effects in the first embodiment can be obtained.
(Sixth Embodiment)
[0094] A sixth embodiment is described next.
[0095] In the description below, description of a portion common to the first embodiment
is omitted, and a portion different from the first embodiment is principally described.
[0096] FIG. 7 is a diagram showing the overall configuration of a power system including
a variable-speed pumped storage power system according to the sixth embodiment. FIG.
8 is a configuration diagram mainly showing a functional configuration related to
an overvoltage protection device 20 of the variable-speed pumped storage power system
according to the sixth embodiment. FIG. 9 is a diagram showing an example of an internal
configuration of a reset control locking condition determination unit 27" in FIG.
8. The same reference numbers are given to the same components as those in FIG. 1
to FIG. 3.
[0097] In the first embodiment described above, a fact that the value of the "voltage on
the low-voltage side of the main transformer M" during a period before control is
performed to forcibly release the short-circuit state of the short-circuit device
16 is not smaller than or equal to a voltage specified value that is specified in
advance (or does not fall below the voltage specified value) is one condition for
performing reset control. Whereas, in the sixth embodiment, a fact that an impedance
(a complex impedance having a real part R and an imaginary part X) that is obtained
from a "voltage and current on a high-voltage side of a main transformer M" during
a period before control is performed to forcibly release the short-circuit state of
a short-circuit device 16 is not within a specified range that is specified in advance
is one condition for performing reset control.
[0098] This is implemented by configuring the reset control locking condition determination
unit 27" to be provided with the value of a voltage V
S of each phase detected by a voltage detector 104 on the high-voltage side of the
main transformer M and the value of a current I
S of each phase detected by a current detector 105 on the high-voltage side of the
main transformer M, instead of the value of a voltage V
L of each phase detected by a primary circuit voltage detector 9 on a low-voltage side
of the main transformer M, and by appropriately changing the design of the arithmetic
expression of an arithmetic circuit 32" and the threshold of a threshold processing
circuit 33".
[0099] In this case, the reset control locking condition determination unit 27" is configured
to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage
side of the main transformer M or when a failure has not occurred on the low-voltage
side of the main transformer M or inside the main transformer M (in this case, a fact
that an impedance obtained from a high-voltage side voltage and a high-voltage side
current of the main transformer M during a period before control is performed to forcibly
release the short-circuit state of the short-circuit device 16 is not within the specified
range that is specified in advance), or the reset control locking condition determination
unit 27" is configured to detect that the prescribed phenomenon has not occurred.
In logic circuits L1 and L2, switching control circuits F1 and F2, switches SW1 and
SW2, and the like, whether control to forcibly release the operation of an overvoltage
suppression device 13 will be permitted to be performed is determined based on the
detection result. Control to forcibly release the operation of the overvoltage suppression
device 13 is performed under a condition wherein the phenomenon described above has
been detected.
[0100] The arithmetic circuit 32" calculates an impedance of each phase based on the value
of the voltage V
S of each phase detected by the voltage detector 104 and the value of the current I
S of each phase detected by the current detector 105. As an example, in the case of
a short-circuit failure, an impedance (a magnitude and a phase angle) for each phase
is calculated according to the equations below, and is output.
where
- Za-b:
- impedance between a-phase and b-phase;
- Zb-c:
- impedance between b-phase and c-phase;
- Zc-a:
- impedance between c-phase and a-phase;
- VSa:
- voltage of a-phase;
- VSb:
- voltage of b-phase;
- VSc:
- voltage of c-phase;
- ISa:
- current of a-phase;
- ISb:
- current of b-phase; and
- ISc:
- current of c-phase.
[0101] The threshold processing circuit 33" determines whether the impedance (the magnitude
and the phase angle) of each of the phases falls within a prescribed specified range
A on an impedance complex plane that is formed by an R-axis and an x-axis shown in
FIG. 10. When there is an impedance that falls within the specified range A, the threshold
processing circuit 33" outputs a signal of the value "1". When no impedances fall
within the specified range A, the threshold processing circuit 33" outputs a signal
of the value "0". However, the specified range A is not limited to a range in the
example of FIG. 10. The design of the specified range A may be appropriately changed
according to, for example, the specification of continuous operation requested for
a power plant.
[0102] The specified range A is set from the viewpoint of not hindering the performance
of reset control when a failure has occurred on the high-voltage side of the main
transformer M and of obviating the flowing of a current that may damage equipment
such as the short-circuit device 16 including semiconductor elements in a case where
reset control is performed when a failure has occurred on the low-voltage side of
the main transformer M or inside the main transformer M. Accordingly, the specified
range A is set, for example, in such a way that a failure occurrence point at the
time when a failure has occurred (whether a failure has occurred on the high-voltage
side of the main transformer M or has occurred on the low-voltage side of the main
transformer M or inside the main transformer M) can be identified from an output value
of the threshold processing circuit 33".
[0103] More specifically, when the failure occurrence point is located on the high-voltage
side of the main transformer M, the threshold is set in such a way that a value output
from the arithmetic circuit 32" (namely, the impedance of each of the phases) is outside
the specified range A. When the failure occurrence point is located on the low-voltage
side of the main transformer M or inside the main transformer M, the threshold is
set in such a way that the value output from the arithmetic circuit 32" (namely, the
impedance of each of the phases) is within the specified range A.
[0104] By setting the specified range A as described above, the reset control locking condition
determination unit 27" can detect based on the output value of the threshold processing
circuit 33" that a failure has occurred on the high-voltage side of the main transformer
M (or a failure has not occurred on the low-voltage side of the main transformer M
or inside the main transformer M), or the reset control locking condition determination
unit 27" can detect that a failure has occurred on the low-voltage side of the main
transformer M or inside the main transformer M (or a failure has not occurred on the
high-voltage side of the main transformer M).
[0105] According to the sixth embodiment, also in a configuration using the "impedance on
the high-voltage side of the main transformer M" in locking condition determination,
effects similar to effects in the first embodiment can be obtained.
[0106] In each of the embodiments described above, an example in a case where an overvoltage
protection device is applied to a variable-speed pumped storage power system using
"pumped storage" as energy. However, the present invention is not limited to this,
and the overvoltage protection device may be applied to a variable-speed power system
using another type of energy such as "wind force".
[0107] As described above in detail, according to each of the embodiments, appropriate control
is performed such that equipment is not damaged, and a reduction in the number of
parallel circuits of semiconductor elements that form a short-circuit device, a reduction
in a manufacturing cost, a reduction in the size of a device, a reduction in the size
of a building, and the like can be achieved.
[0108] While certain embodiments have been described, these embodiments have been presented
by way of example only, and are not intended to limit the scope of the inventions.
Indeed, the novel embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in the form of the
embodiments described herein may be made. The accompanying claims and their equivalents
are intended to cover such forms or modifications as would fall within the scope of
the inventions.